1. Field of the Invention
The present invention relates to a resist pattern forming method in semiconductor device manufacturing techniques and, more particularly, to a method of exposing a chemical amplification type photoresist film formed on a semiconductor substrate through a reticle on which a desired semiconductor integrated circuit pattern is drawn or a mask in an exposure atmosphere containing ammonia at a concentration of 2 to 9 ppb and developing the pattern using a developing solution after baking (PEB) to form a rectangular resist pattern.
2. Description of the Prior Art
In a chemical amplification type resist and, more specifically, a two-component system positive resist (e.g., Japanese Unexamined Patent Publication No. 4-44045) containing a protective polyvinyl phenolic resin and a photo-acid generator, or a three-component system positive resist obtained upon adding protective bisphenol A to the two-component system positive resist as a dissolution inhibitor, an acid generated upon exposure is neutralized in the resist film surface region by a basic substance (mainly ammonia) entering the resist film from the exposure atmosphere and deactivated. Since a protective group elimination reaction does not progress in the subsequent PEB (baking) reaction, a sparingly soluble surface layer is generated. For this reason, a T-shaped resist pattern is formed after development, as shown in FIG. 2, resulting in degradation in resolution, focal depth, or dimensional accuracy.
To solve these problems, some methods have been conventionally proposed. For example, the PEB atmosphere is replaced with an inert gas to prevent acid deactivation (Japanese Unexamined Patent Publication No. 4-369211), or processes from pattern exposure to development are performed in an atmosphere containing no gas impurity (Japanese Unexamined Patent Publication No. 6-140299).
Especially, a method of attaching a chemical filter to the coating/developing unit and performing PEB and development under an atmosphere at an ammonia concentration lower than 1 ppb to prevent acid deactivation (Japanese Patent Publication No. 7-142312) is widely used.
As described above, the method of controlling the ammonia concentration in the exposure/development atmosphere to be smaller than 1 ppb using the chemical filter is most popular. Therefore, a demand has arisen for a chemical amplification type resist which is excellent in the environmental resistance and minimizes the dimensional variation even when the chemical filter does not function due to some reason.
However, not only a resist using a tBOC (t-butoxycarbonyl) protective resin but also a resist using an acetal-based protective group which is said to be more excellent in the environmental resistance than the tBOC protective resist, or an annealing type resist (UVII or UVIII available from Shipley) can hardly completely overcome the dependence on ammonia concentration.
If these resists are exposed and developed in an atmosphere containing ammonia at a concentration higher than 10 ppb, the tBOC protective resist has a conspicuous T shape, as shown in FIG. 2, and a system more excellent in the environmental resistance than the tBOC resist degrades in the stand-still resistance after exposure because the ammonia concentration is relatively high although the sparingly soluble surface layer is rarely generated.
In a method of attaching a chemical filter 303 to a conventional coating/developing unit 300 and performing PEB/development in an atmosphere at an ammonia concentration lower than 1 ppb using an excimer laser beam 304 to prevent acid deactivation, an excessive acid 305 is generated and diffused to unexposed portions to induce the protective group elimination reaction. In this case, as shown in FIGS. 1A to 1E, in the tBOC protective resist, the resist using the acetal-based protective group, or an annealing type resist 302, the upper portion of the pattern is cut to result in film reduction to a round shape (FIG. 1E), so no rectangular pattern cannot be obtained.
Reference numeral 301 in FIGS. 1A to 1E denotes a wafer as a substrate.
Especially, in forming a micropattern, the degradation in shape of the photoresist pattern, resolution, focal depth, and etching resistance due to such film reduction is fatal, and it is essential to improve the exposure atmosphere control method.